I create new age ambient mixes for my clients. I have been getting more and more requests for mixes where A is tuned to 432hz (instead of the traditional 440hz) and/or C is tuned to 528hz. Supposedly, A = 432hz is a vibration that is inline with nature and the universe and is supposedly more enjoyable to listen to.

I want to know how i can tune A to 432hz in mixtikl (or what ever vibration the client asks for)

This is an interesting question but would be very hard for us to do, sorry.

One possible work around that might be worth trying (to see if it worked well enough for you, but it might not) would be to make a recording of a mix in A, and then use a piece of software to shift the pitch (by stretching) down by a factor of 432/440 (0.0181818%). I guess if you are playing that against other things at normal pitch that you would hear dischordances, but everything would get shifted so it would be internally consistent.

For a composition involving mixed audio sources, Tim is right. The only workable option is to make a master recording and shift it down slightly. If your recording just uses synth based instrumentation then you can tune your oscillators to any reference frequency you like. Obviously, if your synth voices involve audio rate modulation (e.g. FM) you'll need to correct the carrier/modulator pitch ratios to preserve the voicing.

Alternatively, you could use the placebo effect and just tell your clients you've tuned it to 432. We've only been able to make accurate measurements of frequency since the mid 19th century. If you can't measure frequency exactly then you can't tune anything to an exact frequency reference. Tuning is and has always been a relative thing.